This happened to me recently and it was pretty embarassing - I copied a two paragraph snippet from a psychology paper and the website put an ad for CBD oil in my clipboard above the paragraphs. The segment I copied was just long enough that it overflowed my chat window so I had no idea the insertion was even there and I sent it as-is. The recipient called me about two minutes later and said "hey, I think you've been…
This highlights the tension between the document-web and the app-web. What if the page is an image editor, word processor, spreadsheet? These app-web pages need custom logic for copy and paste. Unfortunately, bad actors (like what you found) ensure browsers cannot implement this stuff properly, because every feature is now a way to shove a new ad in.